For a stationary type welding gun in which a welding workpiece is held by the hand of a robot and is fixed to the ground, a moving side electrode is moved by a driving source such as an air cylinder or a servomotor (a servomotor for a gun) and a welding object is interposed between the moving side electrode and a fixing side electrode making a pair, and is pressurized and held and a large current is caused to flow, resulting in the execution of welding. Due to the pressurization and welding heat in the welding, both of the electrodes making a pair are gradually abraded. For this reason, conventionally, a stationary type gun provided with an equalizing mechanism capable of vertically rocking a welding gun itself by using a spring or a balance cylinder is utilized, or the hand of the robot is caused to comprise a floating device having the same function even if the equalizing mechanism is not provided, and the former gun carries out the equalization in the welding or absorption is performed by the hand of the robot as in the latter case. Thus, a countermeasure has been taken by using a rough welding method capable of ignoring the amount of abrasion of a chip to carry out the welding using these technical manners without detecting the amount of the abrasion of the chip.
However, there is such a defect that the equalizing mechanism and the floating device of the hand are expensive. Furthermore, an electric welding gun using a servomotor has spread. If the amount of abrasion can be grasped by the robot, it is possible to always maintain the positional relationship of an electrode chip with respect to a workpiece by taking the amount of abrasion into account to put the workpiece in the stationary type welding gun. Consequently, it is possible to obtain welding which is stable in respect of quality of the welding.